public function op_plus($other)
{
if ($other instanceof SassColour) {
return $other->op_plus($this);
} elseif ($other instanceof SassString) {
$other = clone $other;
$other->value = $this->value . $other->value;
return $other;
} elseif (!$other instanceof SassNumber) {
throw new SassNumberException('Number must be a number', SassScriptParser::$context->node);
} else {
$other = $this->convert($other);
return new SassNumber($this->value + $other->value . $this->units);
}
}